A seat belt device is a device that is included in a vehicle such as a car and protects a passenger and generally includes a seat belt retractor. The seat belt retractor is a wind-up machine that winds up and houses a seat belt when the seat belt device is not being used and applies a specified tensile force to the seat belt when the seat belt device is being used.
Formerly, a type of a seat belt retractor which winds up a seat belt by the force of a spring has been used; however, since, in this type of a seat belt retractor, a tensile force applied to a seat belt depends on the pull-out length of the seat belt, it has been difficult to apply a suitable tensile force to the seat belt. Thus, a seat belt retractor employing an electric motor has also recently been used. Also, a seat belt retractor has been controlled by a computer included in a seat belt device to apply a specified tensile force to a seat belt. Hereinafter, the computer that is included in the seat belt device and controls the seat belt retractor is referred to herein as a seat belt control device.
The seat belt control device is a kind of ECU (Electronic Control Unit) which is a computer that control car fittings and/or the like. Since an ECU is operated by the power of an in-vehicle battery, an in-vehicle battery voltage may be decreased to make it impossible to start an engine when an ECU continues to be operated in a non-charged state. Thus, there is the technology of shifting an ECU to a sleep mode when a vehicle is in a specified state. The sleep mode refers to the state in which the power consumption of an ECU is lower than that in a normal operation mode and specifically encompasses a standby mode in which an oscillator which constitutes an ECU is stopped, a slow clock mode in which an operation is performed with a low frequency oscillator different from that in a normal operation, and an intermittent mode in which a computer is intermittently operated at specific time intervals. Currently, a method of reducing power consumption by drastically stopping the functions of a computer and executing only a minimal function (such as a function relating to wake-up) is used. In addition, once a specified wake-up signal is input into an ECU in a sleep mode state, the sleep mode of the ECU is canceled and returns to a normal operation mode.
In a motor drive device that drives the electric motor of a seat belt retractor, a function must be protected from an increase in the temperature of a transistor. Thus, there is a technology that performs motor drive limitation in order to suppress an increase in the temperature of a transistor to a specific value or less.
Patent Literature 1 discloses a motor control device for a seat belt retractor, which confirms, prior to driving a motor beforehand, that drive limitation is not performed during driving a motor, and drives the motor only when there are no problems. The motor control device for a seat belt retractor of Patent Literature 1 compares a counter value varying depending on variation in the temperature of a switching element with a preset threshold value and performs drive limitation when the counter value is more than the preset threshold value.
Patent Literature 2 discloses a seat belt control device which can precisely detect the overheated state of a drive circuit. Patent Literature 2 discloses the technology of interrupting power supply from a power source to protect the drive circuit from being overheated when the overheated state of the drive circuit is detected. Circuit protection means according to Patent Literature 2 is, for example, hardware such as a temperature-sensitive field effect transistor.
Patent Literature 3 discloses an occupant restraint device wherein, when a motor is in an overheated state, a physical sensation alarm (first alarm) is stopped, the operation of an overheat protection function is avoided even at a penalty in the physical sensation alarm, and an occupant can surely be restrained at the time of important occupant restraint (second alarm), from the viewpoint of occupant protection. The overheat protection function according to Patent Literature 3 is hardware such as a thermal protector. The thermal protector is a device for preventing burnout which occurs when the coil temperature of a motor is more than a specified heatproof temperature.